U.S. Pat. No. 3,824,002 issued to Terry D. Beard entitled "Alternating Current Liquid Crystal Light Valve" and assigned to the present assignee discloses the basic principles of operation of an alternating current liquid crystal light valve which requires that a photoconductor be impedance-matched to the liquid crystal, the photocapacitance of the photoconductor being modulated in response to an input light.
U.S. Pat. No. 3,976,361 issued to Fraas et al entitled "Charge Storage Diode with Graded Defect Density Photocapacitive Layer" and assigned to the present assignee discloses a different photoconductor for a similar light valve. The photoconductor consists of a charge storage semiconductor diode with a graded band gap layer increasing the optical absorption coefficient of the region near the rectifying junction to permit the storage of charge.
The aforementioned copending U.S. patent application Ser. No. 792,842, filed on May 2, 1977, discloses the use of a space charge depletion region to transfer field guided minority charge carriers representing signals from one surface of a substrate to the opposite surface using the charge depletion region as a transfer medium. The charges are DC driven and the depletion region in one embodiment is formed by reverse biasing a rectifying junction. As it was indicated in that application, minority charge carriers representing signals can be generated or injected into the storage and transfer medium through a variety of different means. One such means that was disclosed in that application, was the photogeneration of charge carriers inside the transfer medium. One application of the concept disclosed and claimed in the aforementioned patent application is in DC liquid crystal light valves.
The aforementioned copending U.S. patent application Ser. No. 796,641, filed on May 13, 1977 discloses and claims a structure wherein a CCD input register is used to accept and store the input charge and then transfer it out to activate a light display medium such as a liquid crystal layer. The charge packets, after their release from the control of the CCD control voltages, diffuse through a thin epitaxial layer and reach a relatively thick space charge depletion region through which they transfer under the influence of an electric field to the opposite side of the substrate. Such a structure can be used as a charge transfer medium for a DC liquid crystal light valve.
Some of the present photoactivated liquid crystal light valves, made according to the teachings of U.S. Pat. No. 3,976,361 use a thin film of cadmium sulphide which is driven with alternating current. The photodetector acts as a light activated voltage gate. The thin film structure is designed to accept the major portion of the drive voltage when the photoconductor is unilluminated; the portion of the voltage that falls across the liquid crystal is below the threshold for activation of the liquid crystal electro-optic effect. When light falls on the photoconductor, its impedance drops, thereby switching the voltage from the photoconductor onto the liquid crystal and driving the liquid crystal into its activated state. Due to the high lateral impedance of the thin films there is very little spread of the photogenerated signal and of its concomitant liquid crystal electro-optic effect. As a result, the light activation process is a high resolution process, so that the device can accept photographic quality images for transfer to an intense beam of light.
The aforementioned copending U.S. patent application Ser. No. 808,224, filed on June 20, 1977, discloses the use of a single crystal high resistivity substrate which is photosensitive under AC excitation to generate charge and then transfer it under the influence of an AC electric field across the substrate which is depleted of its mobile charge carriers during one portion of the AC bias cycle. The means for depleting the substrate of its mobile charge carriers during a part of the bias cycle is an MOS type of a capacitor formed on a surface of the substrate. The transverse and spatially uniform electric field of the totally depleted MOS capacitor is used to collect photogenerated minority carriers and to provide a spatial resolution by means of field focusing during the depletion phase of the applied AC voltage.
The aforementioned copending U.S. patent application Ser. No. 834,856, filed on Sept. 9, 1977, discloses the use of a conducting microgrid in a portion of an AC liquid crystal light valve semiconductor substrate. This microgrid is a microchannel grid structure which is formed in a predetermined portion of the substrate and has the same type of conductivity as the bulk substrate except it has a higher conductivity. When the substrate is depleted of its mobile charge carriers during a portion of the AC cycle this microgrid is also depleted of its charge carriers and is left with a high concentration of immobile charge carriers of the same polarity as the minority charge carriers of the substrate. When the signal-representing charge carriers move through the substrate they are repelled by these immobile charge carriers of the microgrid towards the center of the light valve cell. Thus, this microgrid focuses the signal charges and improves the resolution of the light valve.
The inventions disclosed and claimed in the aforementioned patent applications and patents represent truly significant advances in this art as explained in detail in said application. Our present invention extends the developments in this area of technology and provides an AC light valve with an alternate readout structure.